A balun is a transformer that can convert electrical signals that are balanced to signals that are unbalanced, and vice versa. For example, the balanced signal may be balanced about a ground (i.e. differential) while the unbalanced signal may comprise a single-ended signal. Moreover, the balun may be used to match couplings between connections of varying impedances.
One typical application for a balun is a dipole antenna feed structure. In particular, the balanced load of the dipole antenna is center fed with a coaxial transmission line, which is unbalanced due to the differences between the inner and outer conductor. More specifically, the signals in the inner and outer conductors of the coaxial transmission line propagate differently since they travel paths of different resistances. The transmission line application is well suited for one common example of a balun, i.e. the transmission line balun. Typically, this balun may comprise a ferromagnetic body, such as a toroid or bar, and the transmission line is wrapped around the ferromagnetic body. In coaxial applications, such as antenna feeds and PC cable connections, the donut shaped ferromagnetic body surrounds the transmission line.
Coaxial cable has become ubiquitous, yet the unbalanced nature of the coaxial transmission line may suffer from the unwanted effect known as common mode current. The common mode current is energy that travels on the outer surface of the coaxial cable outer conductor. This common mode current may cause undesirable interference, and reduce transmission efficiency. The typical balun acts as a “choke” and impedes flow of this common mode current, i.e. a balun choke.
In some applications where the antenna is mounted to extend from a largely metallic chassis, the common mode current passes through to the metallic chassis. In these applications, the metallic chassis may operate as a poor ground plane.